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Flooding

An obstruction of water can occur on drained plots. This can occur immediately after the construction of the drainage, but can also occur a few years later. Puddle forming on plots can also occur after leveling work has been carried out. These flooding’s can often lead to complaints about the functioning of drainage. It is always easy to ascertain that the actual cause is not simply due to the drainage. There are a number of possible causes.

In this chapter we will treat:
Topsoil
Soil structure
Surface per drain
(too) high requirements
(too) little drainage
Measuring groundwater level

Topsoil

A possible cause of flooding can be that the topsoil is to compact. This can have an artificial cause or occur naturally.

Artificial compaction

Artificial compaction has become increasingly common in recent years. The cause is often the increasing and intensive use of the plots, with heavier loads, and under conditions vulnerable to the topsoil. The vulnerability increases with more plowing and leveling. Due to the shrinking of the topsoil and the submerging or decaying of the sod layer, the damping of the sod has disappeared. There is an increasing risk of structure deterioration if you are sowing with a grass species that does not belong to the turf for the second time. An important note on clays soil is the chance that a heavier clay layer ends up in the new sod plays. The decrease in the organic matter content in the sod layer (0 to 5 cm) also reduces the ‘resilient’ character of the sod. This is evident from soil analyzes for determining soil fertility. On heavy clay soil, the topsoil must therefore be preserved as much as possible during leveling work. Otherwise, you will lose too much organic matter on the top centimeters. Furthermore, in the event of stagnation of water at ground level, the plow should not be used too quickly. If possible, sowing with turf formers is important for a new turf with sufficient resilience. Due to swelling and shrinkage, a natural recovery can occur when drying.

Slacking of soil

Fig. 1: slacking of soil cause puddles

Natural compaction

When the topsoil consists of loamy sand, puddles form easily. With an open turf, the top centimeter of the turf can easily slam shut an overflow with an intensive rain shower. In contrast to clay soil, no swelling and shrinkage occur here. Therefore, natural recovery is much slower than on clay soil, or will not occur at all.

Detection of a compaction

Detecting compacted topsoil can both be done visually and through soil investigation. The behavior of the groundwater level can also be monitored over a certain period. An ‘open’ sod, possibly in combination with a (slackened) crusting, is a sign of compaction. Compacted topsoil can also be detected as follows. Dig a hole about 25 cm wide and about 50 cm long and more than a stitch deep. Then cut another side out of the hole of around 10 cm thick. If there is compacted topsoil, the ‘slab’ of soil will fall apart in layers. Even if there are sharp fractures visible instead of a crumbly structure, there is compaction.

Measures

Topsoil that has been compacted by use or naturally can be restored. This can be done through a well-executed operation. If the cause is a one-time incorrect use, such as leveling, tracking, or trampling, the effect of the treatment is permanent. A Naturally compacted topsoil can easily recompact after a treatment. As long as the sod has not (re)formed, this can happen after every heavy rain shower. This compaction can be eliminated in various ways. Several machines have been developed for this purpose, which have varying results after use. There are, for example, differences in costs and in realizing effect. Some machines have a favorable effect on loosening but damage the sod considerably, such as the sharp subsoiler. However, this damage is already largely repaired during a subsequent cut, but only if the soil is not too heavy. Other machines cause less turf damage, but also have a less favorable loosening effect. (Shae-aerator, adapted sharp subsoiler with a roller, vertical drain) A machine like the Paraplow is a favorable exception; with proper adjustment, the damage is limited, while the loosening effect is significant. If there are high costs associated with the operation, plowing and reseeding should be considered.

slacking limits the use

Fig. 2: Slacking limits the use of the land

Soil structure

Malfunctioning of drainage can also be the result of a failure in the soil structure.

Due to a compact soil, water runs to the drains badly.

Fig. 3: Due to a compact soil, the water runs to the drains badly.

Causes

A poorly permeable layer can naturally occur under the drains. The consequence of this is that the supply of water stagnates at the bottom of the drain. the drains will therefore have to be closer together. If this is not taken into account at the time of construction of the drainage, there is a risk of flooding. When there is a poorly permeable layer in the profile above the drains, the drainage will function poorly. This can be the case, for example, in peat soil, in which one or more layers of heavy clay are enclosed in the peat layer. These have been deposited in phases between the peat formation and have had no chance to mature due to inclusion in the moist peat. These clay layers are sometimes only a few centimeters thick, but they permanently disturb the vertical water movement in the soil. When the profile as a whole is very layered, the vertical water movement is often far from optimal due to varying capillary properties of the soil layers. This can result in the temporary occurrence of puddles on the plots. The entire profile can also be poorly permeable. If, for example, boulder clay already occurs above the drains or if the poorly ripened (soft) clay substrate already starts above the drainage depth, the drainage effect is usually poor.

Measures

If the preliminary drainage investigation already shows that the total profile is poorly permeable, you may ask yourselves if whether drainage is a useful matter. If you do decide to drain, because the poorly permeable profiles only occur in some places in the plot, then construction with a chain excavator is preferable. This way, the soil at the location of the drainage trench is excavated and the trench itself can still fulfill a drainage function. In the case of a strongly layered profile, you can consider using soil preparation to eliminate the disturbing effect of the soil layers. If this concerns an enclosed clay layer in peat soil, it will not be without risk and the effect may not be as great. Good results can be achieved with well “miscible” layers, such as clay with sand or sand with sand. If the faults in the profile are detected in time, the profile processing can take place before the drains are laid. If the malfunction is only discovered after the installation of the drainage often as a result of complaints, the process can still be carried out. For ‘safety reasons’, machining in the longitudinal direction of the drains are preferred. Tumbling at right angles to the drain direction can have advantages in connection with the “supply” of water to the drains, but increases the risk of damage to the drain pipes.

Surface per drain

In the event of flooding, the surface may be too large per drain. the quantity of water is than to large in relation to the diameter. A larger drainpipe of a smaller drain distance would prevent this nuisance. When this occurs, the field portion furthest from the power tube is the wettest part of the field. Moreover, it is swampier between the drains than close to the drains.

Measure

In practice, the construction of intermediate drainage is the only way to prevent flooding. This is shown in more detail in figure 8.

Too high demand

People may have too high expectations before the construction of the drainage. If, for example, it is expected that after the construction it will be possible to drive in the plot throughout the winter, expectations will not be met. After all, the requirement for drainage on grassland is that the average highest groundwater level must not rise above 30 cm below ground level. This does not mean that this situation will never occur. (table 5). In arable farming there are heavier demands than on grassland. For more intensive crops, such as coarse horticulture or flower bulbs, the requirements are even more stringent. High demands are also set for areas for intensive use, such as recreation and sports fields. It is therefore not inconceivable that disappointment is based on expectations that are too high concerning the use of the plot, while the drains themselves function perfectly.

Measure

By accepting that the requirements were too high, the existing situation can also be accepted. It is also possible to achieve higher requirements by choosing a smaller stand. In practice, this means installing intermediate drainage. However, this is a doubling of the dewatering capacity. Sometimes this increases the risk of damage due to drought.

Slacking and tearing

Fig. 4: For tearing to happen after slacking, a long dry period is necessary.

Too little drainage

When puddles occur, there are a few options to see what causes this. The optimal drainage result does not have to be achieved every time of the year. As mentioned earlier, the requirements may not be met several times a year. We call this the exceedance frequency. Table 5 shows how often a certain groundwater level can occur at a certain discharge when the drainage has been lad at a discharge capacity of 7 millimeters per 24-hour period. When there is prolonged puddles on the plot, one may wonder whether the criterion is being met. At first, the discharge of the power tube can then be checked.

Drain power tubes

If the power pipes flow with a substantial drain but puddles remain on the plot for longer periods, the surface area per drain may be too large. If the power rube drains very little, the discharge can be measured and converted into millimeters per 24-hour period. It is therefore necessary to measure how long it takes before one liter of water has been drained. The discharge can then be calculated using the following formula. 86.400 (drain length x drain distance x number of seconds needed for 1 liter) = the drain per 24-hour period in millimeters.

Example

The drain length on a plot is 200 meters and the drain distance is 10 meters. It takes 10 seconds for one liter of water to flow out of the power tube. According to the formula, the discharge is then: 86,400: (200x10x10) = 86,400: 20,000 = 4.3 mm.

The discharge standard for grassland is 7 mm, with a groundwater level the middle between the drains of 30 cm below ground level. For corn plots and arable farming, a standard of 50 cm applies in the middle between the drains with a discharge of 7mm. the example shows that the discharge standard is not reached. This may be the result of an already lower groundwater level than the 30 cm or 50 cm associated with a discharge of 7 mm. The discharge can also be larger. This normally occurs when the groundwater level between the drains is higher than 30 cm or 50 cm below ground level. It is therefore important to include the groundwater level in the assessment. After all, the main purpose of drainage is and remains to influence the groundwater level.

Measure groundwater level

The groundwater level as measured in a pipe or hole is a surface that does not occur in the soil. Between the soil particles in the soil, there are spaces, the so-called pores. These are, viewed from top to bottom, filled with air, filled with air and water, and completely filled with water. (see figure 9). The groundwater level measured in a pipe or hole is where all pores are filled with water. We can measure these groundwater levels through boreholes and monitoring wells.

Drill holes

A drill hole can be made to measure the behavior of the groundwater level for several days or weeks in a row. This gives a reasonably reliable impression of the rise and fall of the groundwater level. Drill holes are less suitable for measuring a longer period. In the meantime, they can be damaged or (partly) collapse. The risk of collapse depends on the type of soil and soil structure as well as on the fluctuation of the groundwater over time.

Monitoring wells

Monitoring wells are needed to measure the groundwater level over a longer period. These often consist of plastic tubes and can differ greatly in diameter and length. Usually, the diameter varies from two to four to five centimeters. There are tubes which are provided with perforations over a fairly great length around which a nylon stocking is attached. Here the groundwater can flow over the entire length of perforations in the pipe. This can mean that water flows in from highly permeable layers that occur high in the profile. If you are measuring the groundwater level in the pipe, it is not known from which depth the groundwater originates. To gain insight into this, it is possible to provide the monitoring wells with perforations only over a limited length.

To measure the water level in boreholes or monitoring wells, it is necessary to use a level gauge. These can be used to a depth of up to 10 meters. For greater depths, there is also a gauge light (up to about 50 meters) or a gauge with sound signal (up to about 200 meters deep). The water level can be recorded incidentally, for example, once a day, per week, month or quarter, but also continuously. So-called divers are placed for this, which can be read remotely.

After making the borehole or installing a monitoring well, you should wait at least 24 hours before measuring for the first time. The location of the boreholes and/or monitoring wells depends on which data is desired. To check the influence of the ditch level in a plot, an observation can be made at a distance of, for example, 2, 10, and 50 meters from the ditch. (figure 10 a) Figure 10b shows that the measurement can also take place at a fixed distance from the ditch, right next to a drain. This way the course of the groundwater can be measured near a drain. The influence of the drain length to the end pipe can also be examined. In figure 10c the setup shows that the influence of the drain on the groundwater level can be measured. A reasonable impression of the functioning of ditches and drains are shown in figures 10a, 10b, and 10c.

Examples of flooding and application of the control

Puddles in drained lots may or may not have been caused by compacted topsoil. In addition to a possible observation via the soil profile, it is also possible to derive from the behavior of the groundwater level. This is one of the purposes of placing a monitoring well, or boreholes can be made just around the pools and it is also possible to make them from drain to drain as shown in figure 10. A day later it can be determined whether the topsoil is compacted. For example, if the groundwater around the ponds is clearly below ground level (for example, more than 30 cm), then there is compaction. Besides, the influence of the drains is noticeable due to differences at the location of the drain compared to the center between the drains. Even when there are puddles up to the edge of the plot while the ditch level is, for example, more than 1.00 meters below ground level, there is almost certainly compaction. If the groundwater level is the same as the water in the puddles, the drainage will not function optimally, unless there is a substantial discharge from the end pipe at that moment (figures 11A and 11B). The cause may be that the total profile has been compacted, but also that the drainage has been disturbed.

By examining the most recent use of the plot and assessing the soil profile, the occurrence of soil compaction can be properly estimated.

When it is a drainage failure, puddling only occurs very locally. After all, a drainage system that has always functioned well is rarely disrupted across an entire plot. If there are puddles shortly after the drainage has been installed, this could be a malfunction in one of the drains. Placing monitoring wells or making boreholes from drain to drain and / or along the length of the drain at a fixed distance to the end pipe can clarify this. During all this, the discharge is also checked.

If there is a suspicion of a malfunction, the drain can be punctured, flushed, or measured. Piercing is the simplest form of control. Flushing is a combination of checking and cleaning in one operation. Measurement provides insight into the location, accessibility, and elevation. If a fault is found in this way, the problem can be solved, for example by excavating the fault or by installing a completely new drain.

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